This invention relates to refractory mixtures. More particularly, it relates to a refractory mixture capable for use as a mortar or for castable refractories and having enhanced corrosion resistance to molten aluminum.
In the processing of metal such as aluminum and aluminum alloys for casting or purification, the metal is generally melted in a furnace and conveyed in troughs lined with suitable refractory material to inhibit attack by the molten aluminum. Calcium aluminate is known as a good refractory material from the standpoint of corrosion resistance to molten aluminum. However, as discussed in my parent applications, calcium aluminate has a high thermal conductivity and a high thermal expansion coefficient. Therefore, the use of calcium aluminate with fused silica is sometimes preferred to overcome such difficulties.
My parent applications discussed the problems of metal attack by molten aluminum on silica-based refractories and the prior art attempts to overcome these problems using boron oxide additives. It was pointed out that the prior art incorporated the boron oxide into the mixture by providing a fused or glassy matrix of calcium oxide, boron oxide and aluminum oxide. For example, McDonald et al U.S. Pat. No. 2,997,402 teaches a refractory which comprises a homogeneous vitreous product and a refractory aggregate wherein the homogeneous vitreous product is a fused mixture of calcium oxide, boron oxide and aluminum oxide and the refractory aggregate is principally a mixture of aluminum oxide and silicon dioxide in comminuted form. McDonald et al also suggest that up to 15% by weight of an additional metal oxide component can also be incorporated in the glass such as the oxides of magnesium, barium, beryllium, zirconium, zinc, vanadium, silicon, chromium and molybdenum.
Rubin et al U.S. Pat. No. 3,471,306 mixes bond-forming components containing Al.sub.2 O.sub.3, B.sub.2 O.sub.3, and CaO with a pre-calcined granular grog containing SiO.sub.2 in excess of Al.sub.2 O.sub.3 to form a calcium boroaluminate bond in situ.
While the use of such materials can result in satisfactory mixtures, fritted or glass matrices comprising calcium oxide, boron oxide and aluminum oxide are not easily formed. For example, in the formation of the homogeneous vitreous product of McDonald et al care must be taken to minimize the water solubility of the frit wherein the boron oxide and the calcium oxide might otherwise be leached out. On the other hand, the formation of a calcium boroaluminate bond in situ as in Rubin et al is not easily achieved because of added complications in firing due to the presence of the aggregate (e.g. one cannot melt the bond-forming components as in McDonald et al to ensure formation of a homogeneous stable mixture).
It is therefore an object of the invention claimed in my parent application and in this application to provide a refractory mixture wherein a boron oxide-containing material is used in the form of a zinc borosilicate glass frit having a low content (less than 0.5% by weight) of impurities such as alkali and alkaline earth additives which could otherwise be leached out of the frit in aqueous systems commonly used to form refractories.